Centrifugal pumps are used in a variety of industries to pump fluids. Slurry-type centrifugal pumps are used to process fluids which contain solid materials. Centrifugal pumps in general comprise a pump casing through which a drive shaft extends to rotate an impeller within the casing. A seal mechanism is usually provided which surrounds the drive shaft in the region near where the drive shaft emerges from the pump casing to attach to the impeller. The seal mechanism is provided to seal the pump casing to prevent fluid from leaking about the drive shaft and through the pump casing.
In certain applications, the fluid, or slurry, being processed by the pump may contain relatively large or small quantities of air that will naturally collect at the seal and build-up over time. For example, centrifugal pumps are widely used in flue gas desulphurization (FGD) processes to remove sulphur from the flue gases and thereby reduce the incidence of acid rain. The flue gases are scrubbed in a large tank or vessel by forcing the flue gases through a spray of fine limestone particles that are mixed with water to form a slurry. Centrifugal pumps circulate the limestone slurry from the bottom of the tank or vessel to banks of sprays positioned at the top of the tank or vessel. The flue gases enter near the bottom of the tank and exit at the top of the tank.
Air is often forced into the slurry at or near the bottom of the vessel to aid in the chemical reaction of the limestone particles within the slurry and the sulphur particles within the flue gases. Agitators are also used to circulate and mix the slurry and air. Centrifugal pumps, usually having a high flow rate capability, take the slurry feed from the bottom of the tank. Consequently, the feed slurry entering the pumps has a significant amount of air in it.
Air in slurry can cause a variety of problems in centrifugal pumps. For example, higher air content can reduce the density and pressure developed in the pump, particularly if the air is from three to five percent, or higher, by volume. Additionally, the air, being less dense than water, tends to collect around the pump drive shaft near or at the back of the rotating impeller near the stationary pump casing where the mechanical seal is located.
The mechanical seal typically used in centrifugal pumps generally comprises two adjacent seal members, each having a flattened face which abuts the flattened face of the other seal member. One seal member rotates with the pump shaft and impeller while the other seal member is stationary. Therefore, one seal face is moving while the other is stationary. The adjacent seal faces are held in close contact by springs and by the internal pressures of the pump when in operation. Maintaining a thin fluid film between the seal faces for lubrication and cooling is critical to seal reliability.
The seal members are made of very hard material, such as silicon carbide, so that the infiltration of particulate matter from the slurry usually does not produce any significant wear in the seal faces under normal conditions. However, when there is a higher volume of air in the slurry being processed, the air can infiltrate between the seal faces and displace the liquid film causing dry spots to form between the seal faces. As a result, the adjacent faces begin to operate or run in a dry condition in absence of lubrication, and friction increases with a concomitant increase in heat within the seal. Microcracks and chipping may form in the seal faces and may cause a rounding of the faces so that more slurry can infiltrate between the seal faces. As larger particulates infiltrate between the seal faces, more wear occurs and the seal mechanism finally begins to leak and fail.
The damage that air in slurry can cause to the seal mechanism is recognized in the industry. It has been proposed, for example, that apertures be formed through the back shroud of the impeller (i.e., that portion of the impeller adjacent the drive side of the pump casing) to allow the high pressure fluid to circulate back to the pump intake or low pressure suction side of the pump casing and thereby take some of the air with it. However, the apertures may become clogged with debris or solids from the slurry, or the flow through the apertures may be insufficient to remove the air, and the benefit that may be derived from the apertures is defeated.
Thus, it would be advantageous in the art of industrial pumps and the processing of slurry with higher volumes of air to provide a system for diffusing or continuously removing air from near the seal mechanism to prevent degradation of the seal as previously described, and to improve pump operation.